Bacterial Genetics & Operons

Transcription

1 Bacterial Genetics & Operons

2 The Bacterial Genome Because bacteria have simple genomes, they are used most often in molecular genetics studies Most of what we know about bacterial genetics comes from the study of Escherichia coli Bacteria have one double-stranded circular DNA molecule, with a little bit of associated protein

3 Bacterial vs. Eukaryotic Genomes E. coli 4.6 million nucleotide pairs 4,400 genes Typical eukaryote: base pairs E. coli s DNA is tightly coiled so it will fit inside the cell most of the DNA is found in the nucleoid region Most bacteria also have a number of plasmids Plasmids are much smaller circles of DNA, each with only a few genes

4 Bacterial Reproduction Bacteria divide by binary fission Before the cell can divide, the bacterial chromosome must be replicated DNA replication begins at a single origin of replication, but goes in both directions around the circle Asexual process: The offspring is produced from a single parent, so they have the same DNA HOWEVER, even though random mutations are rare in individual bacteria, because bacteria reproduce so quickly, there is a lot of genetic diversity in a population of bacteria

5 Sources of Genetic Diversity in Bacteria In eukaryotes, meiosis & fertilization provide for genetic recombination (new combinations of genes) However, meiosis and fertilization do not occur in bacteria Instead, 3 processes are responsible for combining DNA from different individuals Transformation Transduction Conjugation & Plasmids

6 Transformation Transformation is the uptake of naked, foreign DNA from the surrounding environment The foreign DNA is incorporated into the DNA of the bacterium The cell is now considered a recombinant because its genome contains DNA from 2 different sources

7 Transduction In transduction, phages (viruses that infect bacteria) transfer DNA from one bacterium to another Sometimes this can happen as a mistake during the lytic cycle of a virus

8 Conjugation & Plasmids Conjugation is the transfer of genetic material between two bacterial cells that are temporarily joined, via sex pili The DNA that is transferred is usually a plasmid (small, circular, self-replicating piece of DNA that contains only a few genes, separate from the bacterial chromosome) This DNA can give a genetic advantage to the cell that contains it The F plasmid is required for conjugation to occur The cell that gives the F plasmid is F + ; the cell that receives the F plasmid is F - Both cells are F + after conjugation because the plasmid is replicated before the plasmid is passed on

9 R Plasmids & Antibiotic Resistance Sometimes, antibiotics that used to work to kill certain bacteria become ineffective WHY?? Some bacteria have R (resistance) plasmids, which carry genes that cause antibiotics to be ineffective for a variety of specific reasons Why does this matter? Antibiotics will kill bacteria that DO NOT have R plasmids, while those that are resistant to antibiotics will continue to live and reproduce Therefore, the bacteria who have R plasmids and are antibiotic resistant will become more common

10 Transposons- jumping genes Genetic material can also be moved (transposed) within a single cell A transposable element may move within the chromosome, from a plasmid to the chromosome (or vice versa), or from one plasmid to another Sometimes transposable elements are called jumping genes

11 Regulation of Gene Expression Bacteria respond to their environment by regulating their gene expression Bacteria require tryptophan to survive If there is not enough tryptophan in the environment, the bacterium responds by activating a metabolic pathway that makes tryptophan from another compound If, later, however, there is enough tryptophan in the environment, the bacterium switches off that metabolic pathway to conserve resources Light switch analogy

12 Regulation of Gene Expression The metabolic pathway that controls the production of tryptophan can be controlled in 2 ways: 1. The first enzyme in the pathway can be turned off by the presence of enough tryptophan FAST response 2. The expression of the genes that produce the enzymes that make the tryptophan can be turned off by the presence of enough tryptophan SLOWER response OPERON model

13 Operons In E. coli, all 5 genes that code for the production of the enzymes that make tryptophan (when necessary) are all located together Benefit: A single on/off switch can control the whole group of genes This on/off switch, which is part of the DNA, is known as an operator The operator controls the access of RNA polymerase to the promoter, and therefore controls whether or not transcription occurs The promoter, operator, and the genes they control are known all together as AN OPERON

14 trp Operon- Repressible operon The trp operon is turned on normally Therefore, RNA polymerase can attach to the promoter and transcribe the genes However, if there is enough tryptophan, a protein called a trp repressor binds to the operator and blocks the way so that RNA polymerase cannot attach and can t transcribe the genes This is known as a repressible operon because it s usually on, but it can be turned off

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16 Trp Operon

17 lac Operon-Inducible Operon The lac operon differs from the trp operon because it is an inducible operon It is usually off but can be turned on The enzyme B-galactosidase is necessary to break down lactose (disaccharide) into glucose and galactose If there isn t much lactose, there aren t many enzymes In the normal state, the lac repressor is bound to the operator, causing the gene to be off When lactose is added to the environment, however, an inducer inactivates the repressor, turning the gene on

18 Lac Operon

19 Lac Operon